(a) Technical Field
The present invention relates to a functional reinforcing filler including inorganic particles surface-modified with an alkenylsilanol obtained from hydrolysis of an alkenylalkoxysilane compound.
(b) Background Art
Since the mid-20th century, silanes having organic functional groups have been widely used to enhance adhesion between silica and polymers or to improve compatibility. However, a silica-filled rubber composition does not have a suitable reinforcing effect and an excellent improvement in physical properties can be attained only when a sulfur-containing coupling agent such as 3-mercaptopropyltrimethoxysilane (MPTMS) is used. Further, the mercaptoalkyltrialkoxysilane coupling agent has offensive odor and, when it is mixed with a polymer composition, processability is degraded since the prevulcanization time is greatly reduced due to the presence of highly reactive thiol (—SH) group.
In the early 1970s, bis(alkoxysilylalkyl)polysulfides [(RO)3SiCH2CH2CH2SxCH2CH2CH2Si(OR)3] were developed [U.S. Pat. No. 3,842,111, U.S. Pat. No. 4,384,132, U.S. Pat. No. 4,507,490]. And, in the early 1990s, Michelin announced the “green tires” using bis(triethoxysilylpropyl)tetrasulfide (TESPT) [Eur. Patent EP 0501227, U.S. Pat. No. 5,227,425]. Since then, TESPT has been frequently used as a filler along with silica in order to improve the physical properties of a rubber composition. However, TESPT has a temperature restriction when mixing it with a rubber composition. For instance, when it is mixed with a rubber composition at high temperature, prevulcanization of the rubber mixture occurs because of irreversible thermal cracking of the polysulfane groups. Meanwhile, if the mixing is performed at low temperature, the alkoxy group of TESPT may not be completely hydrolyzed. According to Wolff, S., complete hydrolysis of TESPT is difficult to be attained at low temperature because of steric hindrance [Wolff, S. Kautsch. Cummi, Kunstst 1981, 34, 280]. As a result, the residual alkoxy group exists in the molecule and hydrolysis occurs continuously even after mixing with the rubber composition. It decreases the life span of the rubber mixture as it is released as alcohol from inside the rubber matrix
Thus, highly dispersible silica capable of minimizing the release of alcohol and enhancing the dispersibility of silica is consistently developed by silica manufacturers [U.S. Pat. No. 0,176,852 A1]. However, the procedure of using TESPT and its product thereof cannot be free from the problem of alcohol release. At present, most of silica-containing rubbers use TESPT or MPTMS, and it is quite rare to use a silane having a sulfur-free organic functional group, which allows less improvement in rubber properties, in a rubber mixture for a tire.
It is known that when an alkenylalkoxysilane having a sulfur-free organic functional group is mixed with a rubber together with silica, rubber properties can be improved by using a catalyst such as butyllithium or peroxide [U.S. Pat. No. 0,019,554 A1]. However, in that case, coupling tends to occur with the silane coupling agent rather than the coupling between silica and rubber due to hydrolysis of the chlorine or alkoxy group of the silane, thus resulting in the formation of polysiloxane. This leads to insufficient improvement in the properties of the tire composite and inevitably leads to unfavorable processability and economic loss because of the need of additional use of additives in the manufacture of rubber for tires.
In contrast, alkenylsilane, which is commonly used in the manufacture of a rubber mixture, allows a long prevulcanization time when mixing with a rubber composition and the mixing temperature is not particularly restricted. However, liquid alkenylsilanes, especially vinylsilane and allylsilane, have storage problems because of fast hydrolysis. During mixing with the rubber mixture, they are hydrolyzed quickly, leading to fast condensation with the coupling agent rather than the silica-coupling agent-rubber coupling. Also, a larger amount of sulfur is required as compared to when a sulfur-containing coupling agent, e.g., MPTMS or TESPT, is used. That is to say, since the coupling agent is sulfur-free, solid sulfur or peroxide has to be added. Unless the rubber mixture is mixed homogeneously, the desired improvement in physical properties cannot be attained.